1. Field of the Invention
The present invention relates to a transmitter-receiver having an antenna device and, particularly, to the antenna device of a transmission line type constituted by two lines opposed to each other.
2. Description of the Related Art
In general, an antenna device of transmission line type has a line placed above a planar conductor with a spacing provided between the line and the planar conductor, and a signal is fed between the line and the planar conductor. Ordinarily, characteristic analysis on such an antenna device is performed by using a mirror-image line emerged in such a position that the mirror-image line and the actual line are symmetrical about the planar conductor, and the two lines formed by the actual line and the mirror-image line can be regarded as transmission lines. For this reason, this antenna device is called a transmission line type. This antenna device of transmission line type is known as a transmission line T type, a transmission line M type, a transmission line F type (inverse F type) or the like.
An antenna device used in the field of amateur radio or the like and called “hentena” (see, for example, Japanese Patent Laid-Open No. H9-284028) can be regarded as an antenna having an actual line formed as the mirror-image line in the transmission line M type of device.
The above-described conventional antenna device of transmission line type is formed of transmission lines having a low radiation resistance. In the conventional antenna device of transmission line type , therefore, a feed current several to several ten times larger than that in an ordinary antenna device is required for the antenna elements to obtain the same radiation power as that of the ordinary antenna device. Furthermore, the low radiation resistance provides a large quality factor of the antenna and a narrow frequency band for impedance matching.